Active acoustic transmission loss box

ABSTRACT

The invention relates to noise or sound control achieved by enclosing the noise source in an active enclosure. Arrays of vibration inputs (for example, shakers, piezoceramics, etc.) are attached to the walls of the active enclosure, or loudspeakers located inside the enclosure can be used to excite the sides of the enclosure. An array of error microphones are located in the radiated acoustic field or PVDF strips are positioned on the wall. A controller senses the levels of sound observed at the error microphones or PVDF film and adjusts the oscillating inputs (in terms of frequency content, phase and magnitude) to the active vibration inputs in order to minimize the radiated sound.

BACKGROUND OF THE INVENTION

This present invention relates generally to noise or sound control andmore particularly to the control of radiated sound from vibratingmachinery by enclosing the machinery in what is termed an "active box orcontainer". The purpose of the active box is to markedly reduce theradiation of the sound from the machine to observation points in thesurrounding field, with a very lightweight, compact, non-airtightstructure.

DISCUSSION OF RELATED ART

In many applications the radiation of sound from vibrating machines isan annoying noise problem. One technique which has been used in the pastis to enclose the machine in a high transmission loss (TL) box in orderto reduce the radiated sound (as described, for example, in U.S. Pat.No. 4,715,559, hereby incorporated by reference, herein and in "Noiseand Vibration Control" by L. Beranek, 1988). These conventional boxesattenuate the sound transmitted through their walls by passive means. Inorder that the container be effective, i.e. strongly reduce the sound,it has to be both airtight and constructed from material which has ahigh density and thickness. These two conditions have a number ofpractical disadvantages. For example, the airtight condition impliesthat it would be extremely difficult to build an effective high TLcontainer for applications which require air flow (e.g.a.c. units,compressors, etc.) or piping and wiring connections or ventilation forcooling. These requirements would imply significant holes through whichthe acoustic energy could leak. The high density material condition ofcourse would imply that the box be extremely heavy and large in size, aproblem which is exacerbated as the frequency of sound becomes lower.

Previous work has shown the extremely high potential of using activevibration inputs to structures to reduce the radiated sound from thestructural vibration. Such work is described in "Apparatus and Methodfor Global Noise Control", U.S. Pat. No. 4,715,559, 1987, by C. R.Fuller and "Control of Sound Radiation with Adaptive Structures",Journal of Intelligent Material Systems and Structures, Vol. 2, pp.431-452, 1991, by R. L. Clark and C. R. Fuller. The control inputs canbe in the form of point force shakers or surface strain devices, such aspiezoelectric elements, bonded to the surface of the structure. In orderthat the control approach be efficient and effective, the variable to beminimized has to be the radiated sound from the panel, measured, forexample, by error microphones located in the radiated sound field as inFuller. The controller format can be any control approach which adjuststhe oscillating voltage inputs to the piezoelectric inputs, for example,in order to minimize the radiated sound observed at the errormicrophones. Polyvinylidene fluoride (PVDF) piezoelectric distributedsensors on the surface of a panel have been used in place of microphonesto sense modes of the panel which are radiating efficiently to the farfield such as that described in "Modal sensing of efficient acousticradiators with polyvinylidene fluoride distributed sensors in activestructural acoustic control approaches", J. Acoustical Society ofAmerica, pp. 3321-3329, June 1992, by Clark and Fuller. The work ofClark and Fuller, for example, demonstrates attenuations of the order of20 dB of sound radiated from panels in the low frequencies (f<600 Hz)with only one or two active actuator inputs.

OBJECTS OF THE INVENTION

It is accordingly an object of the present invention to achieve highattenuation of radiated sound from a vibrating machine by enclosing itwith an "active acoustic transmission loss box".

It is another object of the invention to achieve very high global (hereglobal means throughout an extended area of "volume"), of sound with theabove box constructed from very lightweight thin material, or to use thesides of the sound source itself to reduce radiated noise.

It is another object of the invention to achieve very high global soundattenuation with a container that is not airtight, rather it hassignificant air gaps or holes located in the walls of the container.

These and other objects will become apparent when reference is had tothe accompanying drawings in which

FIG. 1 is a schematic of a typical box (in this case rectangular)surrounding a noisy machine. The active inputs, error microphones andPVDF film as discussed above are shown. Also demonstrated is an air gapin the box sidewall.

FIG. 2 is a typical general controller arrangement used to derive thecorrect active control signal, using microphones as error sensors.

FIG. 3 is a typical general controller arrangement used to derive thecorrect active control signal using PVDF film as an error sensor.

FIG. 4 is a schematic of the use of panels to surround a noisystructure.

FIG. 5 is an azimuth plot of typical noise radiation from an enclosurewith and without active control.

FIG. 6 shows a typical noise spectrum at a selected error microphonewith and without control. This result shows control of broadband ormultiple frequencies simultaneously.

SUMMARY OF THE INVENTION

The machine to be quieted is surrounded by an active enclosure. Arraysof vibration inputs (for example, shakers, piezoceramics, etc.) areattached to the walls of the active enclosure, or loudspeakers locatedinside the enclosure can be used to excite the sides of the enclosure.An array of error microphones are located in the radiated acoustic fieldor PVDF strips are positioned on the wall. A controller senses thelevels of sound observed at the error microphones or PVDF film andadjusts the oscillating inputs (in terms of frequency content, phase andmagnitude) to the active vibration inputs in order to minimize theradiated sound. On minimizing the sound at the error microphones or PVDFfilm the radiated sound from the machine is globally attenuated. Notethat the container can be of any shape and material, and can havesignificant air gaps through the walls.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, an example configuration of the "ActiveAcoustic Transmission Loss Box" is shown in FIG. 1 as 10. A machine 11is operating and radiating unwanted noise inside the box. The machinerequires some air flow for cooling etc. as well as piping and electricalconnections and an air gap 23 can be provided. In order to control thesound radiation the machine is surrounded by an enclosure, in this casea rectangular box 12. In the example of FIG. 1, the box 12 is resting onthe machine support base 13 but also could totally surround it. Dampingor absorptive materials can also be added to the box to attenuate highfrequency noise and improve the structural response of the enclosure.The box can be constructed from a variety of materials such as thinsteel, aluminum, etc. In the case shown the box is manufactured from6.35 mm plexiglass and has dimensions 304.8×304.8×406.4 mm. Piezoceramiccontrol actuators such as 13, 14, 15 (type G1195 of thickness 0.19 mmand dimensions 38.1×63.5 mm) are bonded to the center of each panel.Each actuator consists of a piezoceramic element bonded onto each side,co-located and wired in parallel with 180° phase shift. Such aconfiguration produces high vibration of the panels. These elements canbe positioned in various arrays and also embedded in the material ifrequired.

In order to sense the radiated noise field, a number of errormicrophones such as 16, 17, 18 are positioned in the radiated noisefield. The number and location of the error microphones is dependentupon the modal contribution (from the panel vibration) and radiationdirectivity of the noise. Hydrophones may be used in place of errormicrophones 16, 17, 18. A controller 19 is employed which measures theoutput of the error microphones and then constructs an oscillatingcontrol signal of the correct frequency content and phase which, whenfed to the control actuators 13, 14, 15, etc. causes the sound to bemarkedly reduced at the error microphones and other locations. Analternative to microphones is PVDF thin film which can be placed on thewalls in such a way that energy in the radiating modes is sensed. Onepossible configuration for the PVDF strips such as 20, 21, 22 is shownin FIG. 1. Another alternative would be to use accelerometers to sensethe motion of specific points on the enclosure walls.

One particular control arrangement embodies the Filtered-X adaptive LMSalgorithm discussed by Fuller and is illustrated in FIG. 2. Anoscillating reference signal which has the frequency content of thenoise to be canceled is taken from machine 50. This reference signal 51is also highly coherent with the output of the error microphones. Thereference signal is passed through an analog to digital (A/D) converter52 and fed through a number of adaptive filters 53. The number ofadaptive filters is equal to the number of control actuators used. Thearrangement of the adaptive filter is dependent upon the frequencycontent of the noise. The outputs of the adaptive filters is then passedthrough D/A converters 54 and smoothing filters 55. For piezoceramicactuators 57, this control signal is typically passed through a highvoltage power amplifier and then connected to the electrodes of eachactuator. The error signals from the microphones 56 are sampled usingA/D converters and then used in conjunction with the reference signaland a filtered-X update equation in the controller 61 in order to adaptor change the coefficients of the adaptive filters so as to minimize theerror signals from the microphones as far as possible.

In an experimental arrangement to test the performance of such a systemthe noisy machine is replaced with a 165.1 mm speaker 58 positioned in a184.2 m×184.2 m×114.3 m reflex box. Various test frequencies are thenfed to the speaker to generate noise. The reference signal 51a in thiscase is taken directly from the signal 59 driving the speaker. For thistest the control actuators on diametrically opposite panels were wiredin phase, creating in conjunction with a top actuator 60, threeindependent control channels and hence three adaptive filters. Threeerror microphones such as 56 were positioned at a distance ofapproximately 2 m from the box. In this arrangement the air gap 23 shownin FIG. 1 is approximated by raising the box using 25.4 mm blocks ateach corner thus leaving a total air gap of 361.2 cm², giving apercentage open area in the box of 6.5%.

FIG. 5 shows a typical radiation directivity pattern measured around thebox at mid plane and a distance of 1.7 m. The curve 90 labeled "controlon" gives the radiated noise field with control. The curve 91 "controloff" gives the radiated noise field when the control is not activated.It is apparent that the provides a large attenuation of the sound. Whenthe control is turned on, the results of FIG. 5 and 6, labeled "controlon " show high sound reductions of the order of 20 dB at all angles(i.e. global control).

As discussed by Fuller in U.S. Pat. No. 4,715,559 the active attenuationis achieved as follows. The noise source inside the box radiates soundwhich strikes the enclosure walls and causes it to vibrate (at the samefrequency content as the noise source). The vibrating walls then radiatesound away to the exterior free field of the box where it appears asunwanted noise. The active inputs work as follows. The structuralactuators cause anti-vibration in the walls of the enclosure. When theinputs to the structural actuators are adjusted correctly theseanti-vibrations cancel out those vibrations in the box which werepreviously radiating sound, thus leading to global sound reduction. Asin Fuller's patent, not all vibrations (or modes) in the enclosure willradiate sound and thus the active inputs need only cancel thosevibrations (or modes) that are efficient radiators rather thancontrolling all the vibration. This approach leads to a very low numberof control actuators as opposed to totally canceling the box vibration,and is the key to the success of the approach.

An alternative, shown in FIG. 4, is to enclose the noisy structure 80with close fitting panels 85 instead of a free standing enclosure. Inthis case the enclosure panels are attached directly to the sides of thenoise source. If the regions generating noise are localized or if noisecontrol is needed in certain directions, an advantage to this method isthat the need to enclose the entire structure is eliminated. Inaddition, in many cases a more compact enclosure can be constructedwithout restricting airflow needed for cooling. An example of anapplication of this method would be for the reduction of "hum" fromelectrical transformers. Transformer noise is generated frommagnetostrictive forces in the coil and are propagated to thetransformer skin through the oil field and coil foundation.

FIG. 4 shows a cancellation system 80 for enclosing a noisy structurewith close fitting panels. Controller 81 receives a reference signal 82from the structure and inputs 83, from error microphone 84. Actuators 86are located on close fitting panels 85.

Still another alternative shown in FIG. 3 is to place the actuatordirectly on the surface of the noise source.

FIG. 3 shows noise reduction system 70 with active structural controlprovided with a Noise Cancellation Technologies, Inc. controller 71 andpower amplifier 72 having outputs to piezoceramic actuators such as 73,74 and inputs from PVDF sensor film strips such as 75, 76, 77.

An alternative to using structural actuators to anti-vibrate theenclosure walls is to use loudspeakers to generate a pressure fieldinside the box that will produce the anti-vibrations. Combinations ofdifferent sensors such as speakers and microphones can also be used.

Having described the invention in detail it will be obvious to those ofordinary skill in the art that changes can be made without departingfrom the scope of the appended claims in which

We claim:
 1. An active noise reduction system for canceling a noisedisturbance, said system comprisinga structural container meanssurrounding a noise disturbance with actuator means directly attachedthereto to generate anti-vibrations into said structural containermeans, and a plurality of error sensing means in the radiated noisefield sensing noise radiation external to said structural containermeans and providing error signals, and a reference signal generatormeans for prodding a reference signal containing frequency and temporalinformation on the noise disturbance, and a controller means composingcircuit means for independently controlling each actuator in response tosaid error sensing means and said reference signal to drive said errorsignals to minimum values simultaneously.
 2. The system of claim 1wherein said actuator means are embedded piezoceramic actuators.
 3. Thesystem of claim 1 wherein said actuator means are electrodynamicshakers.
 4. The system of claim 1 wherein said actuator means aresurface mounted piezoceramic actuators.
 5. The system of claim 1 whereinsaid actuator means are loudspeaker means.
 6. The system of claim 1wherein said error sensing means are PVDF film.
 7. The system of claim 1wherein said error sensing means are microphones.
 8. The system of claim1 wherein said error sensing means are hydrophones.
 9. The system ofclaim 1 wherein said structural container means has an air gap therein.10. The system of claim 9 wherein said actuator means are piezoceramicactuator means.
 11. The system of claim 9 wherein said actuator meansare loudspeaker means.
 12. The system of claim 9 wherein said errorsensing means are PVDF film.
 13. The system of claim 1 wherein saidcontainer means has wall means adapted to be close fitting to said noisedisturbance.
 14. The system of claim 13 wherein said actuator meanscomprise piezoceramic actuator means.
 15. The system of claim 13 whereinsaid error sensing means comprises PVDF film.
 16. A method forcontrolling sound radiation of a noise disturbance by active control ofa structural transmission loss container, comprising the steps of:(1)surrounding said noise disturbance with a structural transmission losscontainer; (2) sensing a respective error signal indicative of the noisefield external to said structural transmission loss container soundradiation; (3) generating a reference signal containing frequency andtemporal content of the noise disturbance; (4) actively vibrating thestructural transmission loss container with active inputs in the form ofvibration inputs directly attached or injected into said structuraltransmission loss container via active actuators; (5) controlling thesound radiation at the error signals by adjusting oscillating inputs tothe active actuators by a suitable control law.
 17. The system of claim1 wherein the actuator means generate anti-vibrations only forvibrations of the structural container that are efficient radiators.